The overall objective of our studies is to elucidate cellular and molecular mechanisms important in multistage neoplastic transformation of epithelial cells using rat tracheal epithelial (RTE) cells in culture as an experimental model. Our hypothesis is that some early steps in neoplastic transformation of RTE cells may result from loss in responsiveness of the cells to negative growth regulators. We found that retinoic acid (RA) acts as a negative growth regulator of normal RTE cells, in culture. We also found that induction of early RTE cell transformants by carcinogens was effectively inhibited with nanomolar concentrations of RA. However, once preneoplastic RTE cell clones developed, their cells became rapidly resistant to RA. At 3 weeks after exposure to carcinogen, the 50% growth inhibitory concentration of RA for these transformants was 0.1 nM while at 12 weeks it was greater than 100 fold greater. These data suggest that loss of growth inhibitory mechanisms may be important in early stages of neoplastic RTE cell transformation. The role of oncogenes in late stages of RTE cell transformation was investigated. We found that RNA homologous to the cellular oncogene fms was expressed 5-19 times higher than in normal cells in three of five neoplastic RTE cell lines examined. This increased expression of fms-related mRNA was not due to gene amplification or gene rearrangement. The fms probe detected a 9.5 kilobase mRNA in the neoplastic RTE cells which contrasts to normal rat alveolar macrophages and the human choriocarcinoma line BeWo which expressed a fms transcript of approximately 4 kilobases. We tentatively conclude that, the 9.5 kilobase transcript is related to but distinct from the c-fms gene that encodes the macrophage colony-stimulating factor (CSF-1) receptor. Conceivably these neoplastic cells express another growth factor receptor gene. Studies aimed at identifying this gene using cDNA cloning strategies are currently underway. To identify the ligand of this putative growth factor receptor, binding studies with hematopoietic growth factors related to CSF- 1 are being performed.